C(ELRNTERATA. 133 



the contained cavity becomes in a special sense the gastric cavity. This 

 region of the embryo may be spoken of as the polypite. The nectocalyx 

 grows with great rapidity and soon forms by far the most prominent 

 part of the larva (fig. 75). The true gastric region or polypite (fig. 75, 

 po) continues also to grow, and a mouth becomes formed at its ex- 

 tremity. The aboral end of the original body of the embryo gradually 

 atrophies. 



At the junction of the nectocalyx and polypite the coenosarc 

 becomes formed, and rudiments of a second nectocalyx (nc) and 

 second polypite early become visible ; while a hydrophy Ilium is 

 formed as a bud which covers over the first polypite and tentacle 

 (hph). With the development of the hydrophyllium the first segment, 

 if the term may so be used, is complete. The second segment of 

 which a rudiment is already present as a second polypite is inter- 

 calated between the first segment and the nectocalyces. 



FIG. 76. Two STAGES IN THE DEVELOPMENT OP STEPHANOMIA PICTUM. 



A. Stage after the delamination. ep. epiblastic imagination to form pneuiua- 

 tocyst. 



B. Later stage after the formation of the gastric cavity in the solid hypoblast, 

 po. polypite; t. tentacle; pp. pneumatophore; ep. epiblastic invagination to form 

 pneumatocyst ; Inj. hypoblast surrounding pneumatocyst. 



Amongst the PhysophoridaB there is a considerable range of 

 variation in development ; though the variations concern for the most 

 part not very important points. The simplest type hitherto observed 

 is that of Stephanomia (Halistemma) pictum. The segmentation and 

 formation of a two-layered planula (fig. 76) take place in the usual way. 

 Between the solid central mass of nutritive hypoblast cells and the 

 epiblast an epithelial hypoblastic layer becomes interposed which 

 undergoes a special thickening at the aboral pole. At this pole a 

 solid involution of epiblast next becomes formed, to which a layer of 

 hypoblast becomes applied. The structure so formed is the rudiment 



